微孔洞二氧化鈦薄膜孔洞形貌修飾及其對染料敏化太陽能電池效率之影響

Abstract

染料敏化太陽能電池(Dye-sensitized solar cell, DSSC)是目前極具發展潛力的新一代有機太陽能電池。其中，奈米孔洞二氧化鈦電極扮演著重要的角色，是影響染料敏化太陽能電池轉換效率的關鍵之一。具體來說，奈米孔洞結構二氧化鈦薄膜的對染料的吸附、電子傳輸、電解質擴散等皆有其影響。在這項研究中，利用在二氧化鈦漿料中添加不同分子量的聚乙二醇(polyethylene glycol, PEG)及溶劑，以及不同的聚乙二醇燒除速度修飾二氧化鈦薄膜的孔洞大小和孔隙率。另外，修飾後的二氧化鈦薄膜將封裝成染料敏化太陽能電池，並進一步量測其光電轉換效率及電池內電阻。 二氧化鈦漿料在加入聚乙二醇後，其製備成的二氧化鈦薄膜有較大的比表面積、孔洞大小及孔隙率。若所使用的二氧化鈦漿料為去離子水或以緩慢的速度燒除聚乙二醇，二氧化鈦薄膜的孔洞大小及孔隙率將成會更大。且在聚乙二醇的添加後，二氧化鈦薄膜的孔洞大小分佈則是一雙峰分佈。利用上述方式可以控制二氧化鈦膜的孔洞大小由8.07 nm增加至35.13 nm，而孔隙率由31.05％增加至69.81％。 將二氧化鈦多孔薄膜製備成染料敏化太陽能電池之後可以發現，二氧化鈦多孔電極中具有較大的比表面積可以吸附更多的染料，較大的孔洞大小及孔隙率可以讓電解液擴散更容易，導致更快的染料再生和高的光電轉換效率。增加二氧化鈦多孔電極中的孔洞大小及孔隙率後，染料敏化太陽能電池的光電轉換效率從4.31％上升至5.13％。電化學阻抗譜（Electrochemical impedance spectrum, EIS）分析後可得知，在二氧化鈦/染料/電解質介面上的電子傳遞阻力(R2)和電解液的擴散電阻(R3)的隨孔洞大小及孔隙率增加而增加。此結果可以再次確認從電池效率分析後所得到的結論。 最佳化的奈米孔洞二氧化鈦電極是由添加了15%的聚乙二醇(分子量35,000)的水系二氧化鈦漿料所製備的，其比表面積為59.12m2/g，平均孔洞大小為22.07nm，孔隙為50.87%。以此電極所組成的染料敏化太陽能電池能達到5.13％的光電轉換效率，而其填充因子為0.54，開路電壓為0.73V，短路電流密度則是13.07mA/cm2。

Dye sensitized solar cell (DSSC) is one of leading technology to the next generation of solar cells. The porous TiO2 working electrode plays an important key in DSSCs and affects the conversion efficiency. Specifically, the pore morphology of the TiO2 film affects dye adsorption, electron transport, and electrolyte diffusion. In this study, in order to modify the pore morphology of TiO2 (pore size, surface area, and porosity) to improve the DSSC performance, solvents and PEG in different molecular weights and loadings were added to TiO2 paste, in addition to using different PEG burn out rate after TiO2 films coating. In specific, TiO2 films with different pore morphology were prepared by coating commercial TiO2 nanoparticles (P25) and a scattering layer on FTO conducting glass using doctor-blade technique. The surface area, porosity, and pore size of TiO2 films and the photochemical characteristics of DSSCs with these TiO2 films were examined. Results showed that PEG addition could separate the TiO2 particles leading to modify the pore morphology of TiO2. In butanol based solvent, the surface area and porosity of TiO2 film only increased slightly because butanol is a poor solvent for PEG, and PEG molecules tend to curl in a poor solvent. Even changed the molecular weights, there was almost no difference of molecule sizes in the TiO2 paste. When the solvent was changed from butanol to water, the average pore size was increased from 11.8 nm to 22 nm due to the large solubility of PEG leading full PEG chain extension in the TiO2 particle matrix. And the molecular weights adjusting in water solution increased the pore size from 22 nm for 35k PEG to 30.5 nm for 100k PEG. Because the PEG porogen (pore generator) in good solvent (water) leading the polymer chains of PEG tend to extend well and attach onto TiO2 particles, as the PEG polymer chains are fully extended in water-based TiO2 paste, higher molecular weight shall lead to larger polymer coil size. In addition to using different solvents and molecular weights of PEG, burn out rate is another issue to modify the pore morphology of TiO2. When an additional isothermal bake at 100oC the average pore size increased due to the expansion of PEG polymer chains and possibly some degree of PEG chain aggregation. In contrast, PEG chains were decomposed readily without enough time for chain extension. This yielded smaller pore size (11.8 nm for 35K PEG in butanol system, or 22.0 nm for 35k PEG in water system) under a fast burn-out rate at 400oC. For DSSCs application, the conversion efficiency of DSSC increased with increasing pore size by PEG addition because larger pore size leading electrolyte diffuse more easily in the TiO2 films led to faster dye regeneration, and the electron transport resistance in the TiO2/dye/electrolyte interface (R2) and the resistance (R3) of Nernstian diffusion decreased. In this study, the best conversion efficiency of DSSC reached 5.13% with open circuit voltage 0.73V and short circuit current density 13.07mA/cm2. TiO2 electrode of this DSSC was prepared by 15% 35k PEG loading TiO2 paste in water system and the average pore size was 22 nm.